FIG. 4 shows the basic elements of an end-to-end transmission within the Public Switched Telephone Network (hereinafter “PSTN”). The PSTN shown includes first and second Subscribers, first and second Service Providers, and a Switched Digital Network. Analog Subscriber Loops connect the Subscribers to their respective Service Providers, and the Switched Digital Network connects the Service Providers together. The term “Subscriber” refers to any device that transmits information signals over the analog subscriber loop, such as a telephone, voice band modem, ISDN line, Discrete Multi-tone System (e.g. digital subscriber line), or a computer equipped with one of these devices. The term “Service Provider” refers to a device connected between the analog subscriber loop and the Digital Network. Typical Service Providers include Central Offices and Internet Service Providers.
The Analog Subscriber Loops are conventional twisted pairs that transport analog signals from the Subscriber Equipment to the associated local Service Provider. At the Service Provider, the analog signals are converted to 64 kbps DS0 digital data streams by a channel unit filter and codec, which together implement a bandlimiting filter followed by subsequent analog to digital conversion using a nonlinear encoding rule. The resulting DS0 streams are transported to their respective destination Service Provider via the Switched Digital Network.
At the Service Provider 1, Subscriber's 1 loop signal is first bandlimited. The bandlimited analog signal is then sampled at a rate of 8 ksamples/second, and then converted into an 8-bit digital representation using a nonlinear mapping rule referred to as Pulse Code Modulation (“PCM”) encoding. This encoding is approximately logarithmic, and its purpose is to permit relatively large dynamic range voice signals to be represented with only 8 bits per sample. Exemplary PCM encoding schemes include A-law and mu-law encoding; See S. Haykin, “Communication Systems”, John Wiley & Sons, 1983, 2nd edition, pages 407–438, the contents of which are incorporated herein by reference.
Subscribers 1 and 2 may use a conventional modem to transmit digital data over the configuration of FIG. 4. The conventional modem encodes the subscriber's digital data into a symbol sequence. The symbol sequence is then represented as an appropriately bandlimited analog signal which can be transmitted over the approximately 3.5 kHz bandwidth available on the end-to-end connection. An exemplary modem can include a Digital to Analog converter (i.e. D/A),) an Analog to Digital converter (i.e. A/D), and a hybrid.
To increase the rate of data transmission in the upstream, i.e. from the Subscriber to the Service Provider, it has been suggested that the subscriber transmit using PCM encoding. However, using PCM encoding in the upstream direction faces special problems. In particular, the phase offset between the Subscriber and the Service Provider greatly degrades the performance of PCM encoding in the upstream direction.
Accordingly, there is a need for an apparatus and method that accounts for the phase offset between the Subscriber and the Service Provider.